I would simply do a search for Auckland talent agencies. Most are in Auckland and provide casting for the whole country. It might be harder to find Wellington based agencies online for some reason.

Again, you won't need to be in NZ until very late 2009, but definitely by 2010. Perhaps Guillermo can give a heads up to any fans willing enough to come to NZ and try out as extras, as to when they will be ready to start utilizing them when filming begins.